After a long and bitter campaign, Americans cast their votes on Tuesday in elections that could sweep Democrats from power in Congress and slam the brakes on President Barack Obama's legislative agenda.

Anxiety over the stumbling economy and discontent with Obama have propelled Republicans to the threshold of huge gains that could give them a majority in the House of Representatives and perhaps even the Senate.

"The Democrats' economic plan is not working," said Miami retiree Peter Ruiz after exiting a polling station where he voted for Republicans despite backing Obama two years ago. "We need to try something else."

Opinion polls and independent analysts project Republican gains of at least 50 House seats, far more than the 39 they need to take control and topple Democratic House Speaker Nancy Pelosi from power. The nonpartisan Cook Political Report estimates that more than 90 previously safe Democratic-held House seats are in danger.

Republicans are also expected to make big gains in the Senate, although it will be more difficult for them to pick up the 10 seats they need for a majority.

Polls were open in the US East Coast and Midwest and will start to close at 6pm EDT (2200 GMT), but it will be hours or possibly days before results are known in many crucial races.

All 435 House seats, 37 of the 100 Senate seats, and 37 of the 50 state governorships are at stake in Tuesday's voting.

Obama won office two years ago with the hope he could lead the United States out of a deep economic crisis, but persistently high unemployment and a gaping budget deficit have turned many voters against him.

The public mood gave rise to the Tea Party, a loosely organized conservative movement wary of Obama that backs smaller government, lower taxes and reduced spending.

Obama's signature overhaul of the U.S. healthcare system, a decades-long goal for Democrats, has prompted a backlash.

"They're trying to ruin all the healthcare," said Sharon Krumins, a doctor at Walt Disney World, as she left a polling station in Winter Park, Florida.

Republican control of even one chamber of Congress would likely spark a long bout of legislative gridlock, weakening Obama's hand in fights over the extension of soon-to-expire income-tax cuts and passage of comprehensive energy or immigration bills.

"We can expect very substantial Republican gains leading to super-gridlock for two years," said University of Virginia political science professor Larry Sabato.

Markets opened higher as investors cheered the prospect of Republican gains. "That's biasing the positions toward being long, particularly into areas like finance," said Rick Meckler, president of LibertyView Capital Management in New York.

Republican candidates have pushed an agenda of spending cuts and at least a partial repeal of Obama's healthcare and Wall Street reforms, but Obama could veto their efforts.

Mindful of recent defeats, Republicans have acknowledged that they will need to win back the public's trust if they are returned to power.

"It is not the American people saying: 'We love you, Republicans.' It's the American people saying: 'Republicans, we'll give you another chance,'" Haley Barbour, chairman of the Republican Governors Association and also the governor of Mississippi, said on MSNBC.

Dozens of races are considered too close to call. Candidates in both parties launched a frenetic round of last-minute campaign stops and fundraising appeals on Monday.

The election will determine which party holds the upper hand as legislative districts are redrawn at the state level following the 2010 census.

Voters on Tuesday will also weigh in on a variety of topics: in California, for example, they could approve a measure that would legalize possession of marijuana.

In perhaps the country's most high-profile race, Senate Democratic leader Harry Reid is embroiled in a neck-and-neck reelection fight with Tea Party favorite Sharron Angle in Nevada. The race could hinge on how many voters pick the "none of the above" option on the Nevada ballot.

To win a Senate majority, Republicans need to string together wins in seven of eight tight races in California, Washington, Nevada, Wisconsin, Colorado, Pennsylvania, Illinois and West Virginia.

Democrats mounted a huge get-out-the vote operation to ensure supporters made it to the polls. They were encouraged by their lead among early voters in some key states. Roughly 3 out of 10 voters had already cast their ballots by election day, according to George Mason University political scientist Michael McDonald.

In a final push, Obama was due to give Election Day interviews to radio stations in Los Angeles, Chicago, Las Vegas and Jacksonville, Florida.